1. Field of the Invention
This invention relates to a liquid-cooled, composite piston for internal-combustion engines, particularly for medium-speed diesel engines, comprising a lower part, comprised of a eutectic aluminum-silicon alloy and is connected, e.g., by screws, to an upper part consisting of ferrous material, wherein the upper part is formed on its underside with a concentric bearing rib, which bears on the opposite bearing surface of the lower part and is so arranged that the rib constitutes the radially inner boundary of the annular cooling channel, which is disposed in the upper part behind the top land and behind at least part of the ring-carrying portion, and the rib also defines a central cooling chamber, which is disposed in the upper part and which communicates with the cooling channel by radial coolant bores. The cooling chambers or channels can be open to the interfacial plane.
2. Discussion of the Prior Art
Such two-part pistons are indispensable for high and very high loads. They may be made in various combinations. The head of the piston may be comprised of cast or forged steel or of nodular cast iron having strength properties similar to those of steel. The lower part of the piston consisting of the skirt and the piston pin boss may consist of a cast or forged aluminum piston alloy. The two parts are interconnected by tie rods or screws. All such two-part pistons must be cooled, preferably by a cooling oil.
Since the advent of two-part pistons having an upper part of steel or nodular cast iron and a lower part made from an aluminum piston alloy, wear has been observed between the upper and lower parts of the piston on the surface at which the bearing rib bears on the lower part of the piston. That wear has occurred particularly in the outer portion of the bearing surface and around the clamping screws and is due to radial and axial movements of the bearing surfaces of the two piston parts, which surfaces contact each other in the interfacial plane. These movements are caused by the warping action caused by the deformation of the upper part of the piston owing to the temperature and gas pressure and result not only in wear but involve also the risk of a plastic deformation of the lower part of the piston with a subsequent cracking and a shearing of material. Additionally, the plastic deformation causes the two parts of the piston to approach each other so that the tension of the screws or tie rods is reduced.
It is an object of the invention to provide a piston such that appreciable wear and/or a deformation adjacent to the bearing surface of the lower part of the piston is avoided even after a relatively long running time of the piston.